1. Field of the Invention
The present invention relates to an image forming apparatus that performs inspection of printed matter, a method of controlling the same, and a storage medium.
2. Description of the Related Art
Conventionally, inspection work of printed matter has been performed by visual inspection by an inspector based on his/her experiences. In this case, visual inspection of an enormous amount of printed matter for confirmation puts a large burden on the inspector, and further, there is a high possibility of an inspection error.
Further, to visually inspect printed matter which is output from an image forming apparatus at a high speed is not excellent in working efficiency.
Therefore, there has been a demand for automatically inspecting printed matter printed by an image forming apparatus capable of high-speed small-amount various-type production, in synchronism with printing of the printed matter.
To meet this demand, in recent years, there has been developed an inspection system that makes it possible to detect a defect in print processing, such as dirt, a void, and skew in printing, by reading printed matter printed by the image forming apparatus by a sensor, performing image processing on the read data, and then comparing the read data with print data which is original data.
Further, there has been disclosed a technique in which a finisher having a plurality of output ports is controlled based on a result of inspection on printed matter, and the output port for outputting printed matter is changed between one for printed matter on which printing has been normally performed and the other for printed matter on which a printing defect is detected, thereby making it possible to sort out the printed matter having the printing defect (see e.g. Japanese Patent Laid-Open Publication No. 2005-144797).
Further, some of receipts, securities, and certificates have a background on which is printed a special pattern including characters or images which will appear when copied, so as to prevent them from being easily copied.
Such a special pattern is called a copy forgery-inhibited pattern, and provides a contrivance that prevents an original from being easily duplicated by copying, to thereby realize an effect of preventing copying of the original.
The copy forgery-inhibited pattern is formed by two areas equal in density: an area in which dots remain when copied and an area in which dots disappear when copied. These two areas are substantially equal in density, and hidden characters or images, such as “COPY”, are not recognized at a glance. These hidden characters and images are referred to as the “latent image”.
For example, large dots are dispersed in a latent image part which is an area in which dots remain after copying, and small dots are concentrated in a background part which is an area in which dots disappear when copied. This makes it possible to create these two areas which are substantially equal in density, but are different in characteristics.
In general, in copying an image, there exists a limit in image reproduction capability dependent on an input resolution for reading fine dots in the image and an output resolution for reproducing the fine dots. Therefore, if isolated fine dots exceeding the limit of the image reproduction capability exist in an image, the fine dots cannot be properly reproduced in a copy of the image, and as a result, the copy misses the part of the isolated fine dots. This causes only large dots to appear to make the copy forgery-inhibited pattern noticeable, whereby it is possible to prevent the image from being duplicated.
This copy forgery-inhibited pattern does not function to prevent forgery unless the dots in the latent image part and the background part are correctly printed, respectively. For example, in a case where dots in the latent image part are too small, the dots cannot be read when scanning the image, and the latent image is not left on a sheet to which the original is copied, and hence the copy forgery-inhibited pattern cannot provide a security function.
Further, in a case where dots in the background part are too large, the background dots are also read when scanning the image, so that even the image in the background part is left after copying.
Conventionally, to check whether or not the copy forgery-inhibited pattern is correctly printed, it is necessary to scan a sheet on which a copy forgery-inhibited pattern has been printed and visually check whether or not the latent image has appeared. However, this method is not efficient enough to check a huge amount of originals, and is lower in productivity.
To overcome the inconvenience, when inspecting a print sheet on which a copy forgery-inhibited pattern has been printed using the technique disclosed in Japanese Patent Laid-Open Publication No. 2005-144797, the conventional inspection system uses print data as reference data to be compared with data read by an inspection section.
However, in a case where the copy forgery-inhibited pattern is printed, the print data and the copy forgery-inhibited pattern are printed, and hence if the print data is used as the reference data, the inspection determines that the printed matter is defective. This means that it is impossible to properly perform the inspection.